The present invention relates to a two-dimensional pairing of sliding partners for artificial joint implants, in which a joint element executes a motion, which can be a rotating, rolling or sliding motion or a combination of these, relative to another joint element.
Such artificial joint implants are primarily artificial hip joints, knee joints, shoulder joints, and ankle joints. In general, however, the invention can be used with all artificial joints in which two sliding partners meet with each other.
Generally, one strives to keep the friction as low as possible between two joint elements moving relative to each other. A positive side effect of this is that particle abrasion becomes significantly less with a decreasing coefficient of friction, which benefits the long-term stability of the implant.
After the implantation of the artificial joints, the joint elements are wet by natural joint fluid, the so-called synovia. With joint elements processed for an exact fit, for example joint balls and joint cavities of an artificial hip joint, it can occur that the liquid film between the mutually moving joint elements can wear off and no longer exert any sort of lubricating function. The friction between the joint elements thereby abruptly and significantly increases, whereby the functioning capacity of the artificial joint is impaired, and the abrasion of particles from the sliding partners is fostered. Significant problems with respect to long-term retention of the implant in the patient""s body thereby result.
In the prior art according to British patent specification 1 527 498, solving this problem has already been proposed. Thus, for example, on the boundary surface between the sliding partner in an artificial acetabulum and the artificial hip joint head, depressions are provided in the form of mutually interconnecting channels, in which the bodily fluid should provide for the lubrication of the joint. Quite similar constructions have been made known from German patent 43 36 932 and German published patent application 44 23 020. Due to the fact that the fluid-conducting channels communicate with one another, problems can arise with regard to some channels drying up at exposed points. The problem of uneven lubrication of the artificial hip joint then follows this drying up, whereby its functioning capacity is impaired in a negative manner.
From U.S. Pat. No. 5,462,363, abrasion-proof sliding partners are known, which should also find application in joint implants. Depressions are sunk into the surface of the joint elements, which provide a reservoir for the bodily fluid, which forms a liquid film between the mutually moving elements. The depressions here are generally formed throughout as cylindrical or blind holes. The problem here is that a buffering function of the bodily fluid is only insufficiently exerted in this regard. In the final analysis, this is a consequence of the pressure distribution due to the cylindrical construction of the depression. The synovia can therefore no longer execute its function reliably.
Against this background, the object of the present invention is to improve the above-described two-dimensional pairings of sliding partners for artificial joint implants, such that with high probability the liquid film of the synovia in the gap between the joint elements does not wear off, so that the synovia can reliably exert its function as joint lubricating fluid.
Accordingly, it is proposed that besides the gap for the synovia between the sliding partners, the surface of at least one sliding partner has regularly arranged depressions, which are carried out independently, thus without connection channels. The depressions in the surface of a sliding partner exert a collection function for the synovia, which in turn brings about a buffering action relative to the other sliding partner. The fluid-collecting function of the depressions ensures that a more or less abrupt onset of wearing off of the liquid film between the sliding partners cannot occur. It is constantly necessary to take care that the sliding partners do not directly roll against one another or slide on one another, but instead a film of synovia constantly reduces the friction between them. Fewer abrasion particles are the direct consequence, whereby the long-term stability of the implant is further improved.
The depressions are formed cup-shaped in cross section. In this way, they offer on the one hand a sufficiently large reservoir for the synovia, and on the other the possibility of a good wetting and exchange of the synovia stored in the depressions by the synovia flowing in afterward.
In accordance with one advantageous embodiment, it is provided that the depressions occupy up to 90% of the surface of at least one sliding partner. A relatively large volume of synovia is thereby quasi reserved in a reservoir, in order thus to attain the buffering effect.
The depressions can have a hexagonal opening, thus having in projection a honeycomb appearance. Alternatively thereto, the openings can also be formed round. The selection of one or another possibility depends, among other things, on the overall surface, the surface pressures to be expected, and upon the arrangement on the surface of the joint element in question.
According to a preferred embodiment, the depressions are arranged distributed in the surface of one sliding partner of that joint element which moves in relation to the other, stationary joint element. In other words, the depressions in this case are formed in the surface of the joint element, which executes the rolling on, rolling, rotation or sliding motion relative to the other joint element.
In the case of an artificial hip joint, the depressions would thus be provided in this embodiment on the joint ball, whereas the sliding partner in the acetabulum can be formed with a smooth surface.
The pairings of sliding partners are preferably selected from the group of material pairings: metal-metal, ceramic-ceramic, ceramic-metal, polyethylene-metal or polyurethane/ceramic compound-metal.